Trapezoidal belt variable speed transmissions are commonly used on small vehicles such as snowmobiles, scooters or small cars. Such transmissions substantially comprise a driving pulley, a trapezoidal belt and a driven pulley. The driving pulley is linked to an engine and the driven pulley is usually mechanically connected to ground traction means, such as wheels or tracks.
The main object of using a variable speed transmission is to automatically change the winding diameter of the trapezoidal belt around the driving and the driven pulleys in order to have a maximum torque at low speeds and a reasonable engine rotation speed at high speeds. The sides of the trapezoidal belt are, on each pulley, gripped between two opposite flanges wherein one is fixed and one is movable. At low speeds, the winding diameter of the driving pulley is very small and the winding diameter of the driven pulley is maximum. As the rotation speed of the driving pulley increases, the movable flange of the driving pulley gets closer to the fixed flange and thus forces the trapezoidal belt to wind on a greater diameter.
The changes in the winding diameter is due to the presence of flyweights under the influence of the centrifugal force. The flyweights generate an axial moving force on the movable flange, forcing it to get closer to the fixed flange. The axial moving force is counterbalanced by a helicoidal spring, coaxially mounted around the shaft, generating an axial biasing force opposed to the axial moving force of the flyweights. In addition to the biasing force generated by the helicoidal spring, there is another axial biasing force which is generated by the belt itself because of its winding tension pushing inwardly on the flanges of the conical walls, thereby forcing the movable flange to move away from the fixed flange. Therefore, the movement of the movable flange is a function of the equilibrium between the axial forces of the flyweights, the spring and the belt.
Since the length of the trapezoidal belt is not substantially stretchable, the trapezoidal belt exerts a radial force towards the center on the flanges of the driven pulley in addition to the tangential driving force. This radial force constrains the driven pulley to have a smaller winding diameter. Therefore, the movable flange of the driven pulley moves away from the other until the return force exerted by a spring counterbalances the radial force exerted by the trapezoidal belt. It should be noted at this point that a change in the load also produces a change in the winding diameters of the pulleys, a greater load inducing a greater winding diameter of the driven pulley.
When the rotation speed of the engine decreases, the winding diameter of the driving pulley decreases and the radial force exerted by the trapezoidal belt decreases, thus allowing the driven pulley to have a greater winding diameter.
An example of such a variable speed transmission is disclosed in U.S. Pat. No. 3,266,330.
One of the drawbacks with the conventional driving pulleys is a lack of sensitivity to throttle opening. This problem occurs when the vehicle is travelling at a moderate cruising speed and the driver wants to accelerate full throttle, such as when the vehicle is travelling at 100 km/h and the driver wants to pass another vehicle. The driver then opens wider the throttle of the engine which in return generates a higher torque. Due to the driven pulley reaction, the winding tension in the belt then increases, pushing the movable flange away from the fixed flange until another equilibrium is obtained between the flyweights, the spring and the belt. Therefore, the winding diameter then changes to a lower ratio, sending more torque to the wheels of the vehicle to accelerate it.
The ideal transmission would allow the engine speed to be very low at moderate cruising speed in order to reduce fuel consumption and noise. Yet it would allow very high engine speed whenever throttle is fully opened, thereby getting maximum power from the engine. The problem encountered with conventional variable speed transmissions is that a rise in the engine speed is limited. If the transmission is designed to allow low engine speed at moderate cruising speed, then under full throttle, the engine speed is too low. If the transmission is designed to reach high engine speeds at full throttle, then at moderate cruising speed, the engine speed is too high.
A second drawback of conventional driving pulleys in variable speed transmissions is the response delay when the belt is required to quickly change its winding diameter. This problem is noticed mainly when the vehicle is fully accelerated from standstill. The transmission would ideally stay in its lowest ratio until engine reaches its designed shifting speed. Then the winding diameter of the driving pulley should quickly increase to keep the engine speed nearly constant. The delay in belt response causes the ratio to stay too long in lowest position, thereby allowing the engine to overrun during a short period before it stabilizes at the desired value. This problem is known as "overshoot".
A third drawback of conventional driving pulleys in variable speed transmissions is also due to the belt response delay. When the vehicle is rapidly slowed down, the belt should move rapidly from a large to a small winding diameter in the driving pulley, so that when the vehicle stops, the transmission should be in the lowest ratio, ready to provide full torque output on reaccelerating. Due to delay in belt response, the radial belt pull is less than ideal, and the belt does not return to its smallest winding diameter in the driving pulley at the precise moment where the belt stops turning.